AnyDevice and AnySurface possibly makes incorrect layout assumptions about trait objects

[udoprog]

Reference: rust-lang/reference#1448

Description

This and this cast assumes that the dst cast extracts the (thin) data pointer.

*pointer*
*vtable*

Which while it should mostly match up in practice, is to my knowledge not currently guaranteed by Rust. A different compiler option or future compilers could legally opt to re-order or change the semantics of this behavior.

I have a branch which re-implements these using internal vtables, while also removing HalSurface. I'll be happy to clean it up into a PR once this has been triaged.

[udoprog]

Note that ptr-meta is an unstable feature which provides APIs for decomposing wide pointers, but it is not yet stable nor can I find the proposed casting logic having been accepted into a stable compiler.